. >> reporter: mexican officials immediately contacted the international atomic energy agency. u.s. officials are monitoring closely. the truck was stolen at a gas station just 35 miles outside of mexico city. it is unclear who stole the material. and if they know what they have. but even though the theft occurred hundreds of miles from the u.s. border, there's a reason why the incident is drawing such intense scrutiny. u.s. officials have long been concerned that stolen radioactive material could be used to make a so-called dirty bomb. such a device could not cause a nuclear explosion. but the cobalt, combined with a conventional explosive, could release harmful radioactive elements or something that could at least scare people. >> can they figure out exactly if you have a large quantity of cobalt 60, could you do something environmentally catastrophic? >> reporter: u.s. officials are working hard to track this theft. i just spoke to a senior u.s. official who said there's no reason to panic, but they're taking no chances. >> of course not, because this kind of material has been st